1. Field of the Invention
This invention relates to electro-hydraulic servo controls and methods and apparatus for controlling the operation of a hydraulic injection molding press.
2. Description of the Prior Art
In the development of improved controls for injection molding presses, recent experience, i.e., within the last ten years or so, has included investigation into the possibility of utilizing a computer system to provide computer programmed servo loop controls for injection pressures, packing pressures and velocity of rams and the like that operated upon the "next cycle correction" whereby a computer would be used to analyze a cycle to determine if it did not meet the operating conditions as selected and programmed and, if so, a correction to the system would be entered for control of the next cycle. The capabilities of such computer operated control systems almost dictated from the start that the expense of the computer itself and of the ancillary equipment would be prohibitive and the slow response of the "next cycle correction" proved to be less desirable than a dynamic operation of a system operated in real time.
One of the important factors in controlling a hydraulic injection molding press is "fill time"; i.e., the time from the moment the injection starts until the time that the material is packed into the cavities in the mold. This is important because the optimum condition for injecting a plasticized material into a mold is to fill all of the cavities in the mold while the material is still in a molten condition and will flow freely into each of the cavities. Failure to attain a proper "fill time," which is related to the velocity of the ram, may result in marks on the molded part, voids and nitt lines which may result in a weakening of the portions of the part to be molded. As one further example, the molding of parts to be subsequently plated, dictates that there be no flow lines, voids or nitts because these defects will not properly accept the plating material. As a result of the introduction of "exotic" molding materials as early as the mid-1960's, the "fill time" as determined by the velocity of the ram became more and more important. In fact, the most important of the factors which affect the injection cycle was determined to be the precise control of the "fill time."
However, the material viscosity varies from cycle to cycle dependent upon the nature of the raw material to be plasticized; i.e., the amount of regrind and moisture content. Any change in viscosity will also affect the "fill time." Therefore, injection under a constant pressure type of system, which was and is used on so-called standard machines, results in a variation in the fill time with changes in material viscosity. The only way to precisely control "fill time" is to insure that the conditions that are met as the ram performs its injecting cycle are constantly reproduced during each cycle so that you will always move from what is called shot size or "back" position to the forward or filled position in precisely the same amount of time; e.g., velocity control.
Attempts have been made to precisely control the "fill time" by controlling the velocity of the ram for every step of the way. That is, whether you have a constant or a variable velocity, such as a velocity profile, there must be control over the velocity for every step of the way. One way to accomplish this is to effect a dynamic control under a servo loop-type system which embodies feedback information to force a machine to respond as directed and with great repeatability. Experience with servo flow control valves has resulted in a relatively slow speed system which is unsuitable because, in some cases, the speed of the response of a system exceeds the time of an injection stroke with a resultant undesirable lack of control. Further development led to the use of a servo flow divider system which provided improved response to control signals but still not fast enough and the flow divider valve is not readily adaptable to systems having high hydraulic fluid flow rates. One other important limitation of servo type flow controls and flow dividers is a low tolerance for contamination in a hydraulic system so that a highly efficient filtration system is mandatory and such systems are prohibitively expensive as well as subject to malfunction. Further, many prior art hydraulic system of diverse nature have included the use of one or more adjustable or programmable pressure relief valves to operate as such; i.e., one or more different levels of pressure relief could be used during different cycles of operation. This may be seen in, for example, U.S. Pat. No. 3,932,083 in which a programmable relief valve is applied to an injection molding machine but is operated in an open loop condition and is not intended to be responsive to a dynamic velocity error signal to thereby control the pressure of fluid applied to the piston of a ram in an injection molding machine but only to provide a sufficiently high pressure to allow a flow control to be operable. A further prior art example may be seen in U.S. Pat. No. 3,721,512 in which a system embodies a pressure control valve as, for example, a throttle valve, is controlled by a signal representative of the melt temperature in an injection molding press.
Prior art apparatus has described a multitude of various and sundry ways of controlling the operation of a hydraulic injection molding press in accordance with many conditions which are sensed and used in a control. While past systems have been developed for accomplishing the general purpose of my invention, the systems known to be in existence have two common discrepancies. The first is that expense connected with developing and furnishing the systems has risen as an exponential function of the accuracy desired. Where expense has been a major consideration, the systems have lacked any usable degree of accuracy. Heretofore, the conventional method of controlling the velocity of a machine was by means of a hydraulic flow control valve. This means was not satisfactory because of slow response. An improved means was developed using a hydraulic servo valve. Although this improved the speed of response, it did so at a great increase in expense through greater cost of the servo valve and to the high cost of oil filtration required by the valve.